I. Field of the Invention
The present invention relates to a starting circuit arrangement for gaseous discharge lamps, and more particularly to a circuit arrangement for starting a linear metal halide arc discharge lamp while the lamp is in a semi-conductor inverter circuit.
II. Description of the Prior Art
Gaseous discharge lamps of the long, linear multicomponent, metal halide type are extremely susceptible to cataphoretic effects. That is, the color of the radiated light may vary along the length of the lamp due to the influence of thermal and electrical gradients, and the like, which act to produce a non-uniform dispersion of light emitting cations in the arc discharge. Light feedback, closed-loop, electronic lamp current switching circuits have been devised to counteract these non-uniform dispersions. By controlling relative time of forward to reverse alternations of current in the lamp, it is possible to inject a DC current (superimposed on the AC current) which produces a cataphoretic bias of its own and can be poled so as to oppose the naturally occurring cataphoretic forces. It is possible to construct a circuit operating in such a negative light-feeback control system as described in U.S. Pat. No. 3,700,960 - Lake which will produce a uniform, axial, spectral energy density of light output for such lamps. One of the greatest difficulties facing designers has been in finding circuits which will reliably start such lamps without degrading or causing failure of the electronic switching apparatus needed to control cataphoresis as described.
In addition to being susceptible to cataphoresis effects, multicomponent metal halide lamps are difficult to start. The application of a very high ionizing voltage is required to initiate breakdown leading to a continuous arc discharge. Some have suggested that 8 and 12 Kv is required for starting. This writer, however, has found that, in pulse starting, crest voltages of 30 Kv with a typical rise time of one microsecond are required. This is not surprising since these are the typical parameters for pulse starting long linear arc discharge lamps of similar dimensions. U.S. Pat. No. 3,700,960 - Lake, assigned to the same assignee as the present invention, discloses a metal halide lamp system. With the system as disclosed therein in FIG. 5, a Tesla coil was used as the source of high voltage for initiating breakdown of the linear metal halide lamp. A plurality of bypass switches shown in FIG. 5 and totaling six in number are incorporated in circuit with the lamp 1 between DC input 11 and the circuit ground connection. These bypass switches are a set of mechanical contacts which serve to transfer aside and isolate the comparatively delicate semiconductor switching inverter switches from the deleterious effects of the high voltage starting pulses applied to the lamp during lamp starting.
Typically, the pulse starting voltage is 10 to 50 times greater than the blocking voltage rating of the highest voltage rated power transistors suitable for each leg of a bridge inverter such as that disclosed in the aforementioned Lake patent. One can easily see that the destruction of these transistors would be the general consequence of attempts to start the lamp were it to be directly connected to the bridge inverter without special protective provisions.
Another problem arises in a circuit as disclosed in the Lake patent, this problem also being solved by these six bypass switches. Since a tungsten filament, incandescent lamp is normally used as a resistor ballast in such a lamp circuit, it is necessary to bypass the lamp starting and warm-up current around the transistor bridge inverter. This is necessary because the inrush current of the incandescent lamp ballast and the warm-up characteristics of the lamp would cause a current at the beginning of operation much greater than the transistors could handle and instant destruction would ensue. Such a problem has been resolved by operating the lamp for the warm up period before attempting to switch over to operation by the transistor bridge inverter. Since lamp impedance increases roughly by a factor of 3 during warm up, this places lamp current in a range which the transistor inverter could handle. However, lamp starting by such a method has been rather precarious. Most of the failures have occurred at this time either by catastrophic failure of the transistor bridge or by lamp drop out because of a too long, zero lamp current transition. Furthermore, a serious consequence of this warm up method is the placement of a hard cataphoretic bias on such a linear metal halide lamp. This occurs because the bypass switches place the lamp into a DC operating mode. To counteract this mode after switchover by action of an opposed DC bias from an electronic control circuit to return the lamp to a uniform axial spectral distribution of light takes a very long time: 5 to 15 minutes. This is acknowledged to be fundamentally unacceptable for a practical system.
It is desirable therefore to provide a starting circuit arrangement for such a linear, multicomponent metal halide are discharge lamp which will start the lamp reliably, without need for manual attendance, and be so fast acting that no cataphoretic bias and resultant lamp color imbalance would occur. By the present invention, there is provided a circuit arrangement for starting such a linear, multicomponent metal halide arc discharge lamp while the lamp is in a semiconductor switching inverter circuit.